A20 - macrophyte filters

A20 - Macrophyte filtersA20 - Macrophyte filters
- en - Sanitation and preservation - Treating effluents -
Date de mise en ligne : Tuesday 20 March 2012
WikiwaterA20 - Macrophyte filtersNB. This fact sheet supplements fact sheet A17 "Various ecological effluent treatment solutions"1) What is involved ?Constructed wetlands are involved in several effluent treatment systems presented in fact sheet A17.
They contribute to removing organic matter and impurities from wastewater.
They are sometimes called "constructed wetlands".Their principle involves causing water to flow through a gravel filter on which macrophyte plants (aquatic
plants), such as reeds are grown. Their roots carry oxygen to the soil and create an environment conducive to the
development of water-purifying micro-organisms.
In some way, the process reproduces the natural water purification process in marshes in which wastewater
begins to settle by discharging its solid particles and then undergoes natural physical, chemical and above all
biological treatments favoured by aquatic plants that have the effect of degrading organic matter, removing nutrients
associated with particles and significantly reducing pathogens in wastewater.
As the process lowers the water to the bottom of the hole dug in the ground which contains the filter, use must be
made either of a sloping plot of land or a pump to raise the water to ground level to recover it and feed it to the nextThere are two types of filters : horizontal and vertical. They differ in their feeding method, the water flow direction
and the aerobic treatment conditions.Vertical filters require an intermittent water feeding method and use fairly complex biological mechanisms enabling
them to significantly reduce pathogenic germs and the BOD.Horizontal filters require the water to be pre-treated, for example, by a vertical filter, septic tanks or a small lagoon
to avoid clogging risks. They are more suitable for wastewater containing relatively little solid matter.
These filters are generally incorporated into a set of treatment systems that can be used for domestic water treatment
at family or community level and which can replace cesspools or septic tanks.2) Vertical constructed wetlandsPrinciple
Vertical filters are not fed continuously, but by intermittent scoops (see fact sheet A18 "Systems forspreading wastewater by tanks"). When a scoop arrives, water is distributed on the filter surface and then
percolates evenly. It is recovered from the bottom of the filter, at a depth of about 80cm, by means of a discharge
drain. Sludge will accumulate on the filter surface to a thickness of about 1.5 cm per year. The stems of macrophyte
A20 - Macrophyte filters
plants pierce the layer of sludge and prevent it clogging the filter. Two to three identical filters are generally arranged
in parallel and operate alternately.Feeding and discharge[/IMG/UserFiles/Images/assainir/art-86-0.jpg]Source : Macrophyte plant group and Water treatment
The scoop system must be designed so that each scoop covers the filter with a layer of 2 to 5 cm of water. Ideally,
the filter should receive 6 to 12 scoops a day.[/IMG/UserFiles/Images/assainir/art-86-1.jpg][/IMG/UserFiles/Images/assainir/art Feeding by "closed loop" type overflow via an-86-2.jpg]underground systemH feeding via an overhead system
Here are several ways of spreading water on the filter :
[/IMG/UserFiles/Images/assainir/art-86-3.jpg]Feeding by drains installed on the groundSource : arsatese-loirebretagne association
The two points to watch as regards the spreading of water are :
- the proper distribution of a layer of water covering the whole filter at each scoop
- the flatness of the filter surface : stones should be placed under the water inlets to prevent the occurrence of
holes or washouts dug by the water jets.
When designing the spreading system, provision should be made for the fact that a layer of sludge will settle to the
filter surface (up to fifteen centimetres thick, see "Maintenance"). In particular, the water outlets should besufficiently high above the filter surface.
The water is discharged through discharge drains which should be located at the bottom of the filter without beingvertically in line with a water inlet, so that the water passes through a maximum distance in the filter.A20 - Macrophyte filters
Each filter should have alternating operating and idle phases.
For some small systems (fewer than 100 inhabitants), an operating phase of 3 to 4 days, during which the filter is fed
by scoops, can be alternated with an idle phase lasting the same time. This thus requires two filters : one being idle
In the general case, it is preferred to alternate one operating phase lasting three to four days with an idle period twice
as long. This thus requires three identical filters : one operating while the other two are idle.The main filter element : the filter bed
The filter bed is installed in a hole dug in the ground. It must be sealed to prevent water that has not yet been treated
from seeping into groundwater. It should be between 50 and 70 cm deep.
Top layer : 30 cm of fine gravel (2 to 8 mm in diameter), sometimes placed below a thin 10 cm layer of sand.
Intermediate layer : 10 to 20 cm of gravel (5 to 10 mm in diameter)
Bottom layer : 10 to 20 cm of coarse gravel or pebbles (20 to 60 mm in diameter)
When two sets of vertical filters are used in sequence (as in the case of system No 2 referred to in fact sheet A17 on
the various environmentally sound effluent treatment methods, the top layer of the second filter should preferably be
When the filter is started, 4 macrophyte plants should be planted per m² with their clods. They will multiply during
To ensure the filer's proper operation, ventilation shafts connected to the evacuation drains must be installed,
making sure that they emerge high enough to prevent water from going down this way. (see diagram on page 1).
Evacuation drains must be installed at the bottom of the filter without being vertically in line with a water inlet, so that
the water passes through a maximum distance in the filter.Maintenance
Macrophyte plants should be pruned once a year, leaving a height of 30 cm to prevent plants from being submerged
by the scooped water. If weeds grow in the filter, they should be pulled out by hand as soon as possible and removed
The filter's proper operation should be monitored regularly to ensure in particular that the filter does not become
saturated and that the drains do not become clogged.
As a guide, sludge normally accumulates on the filter's surface at a rate of about 1.5 cm per year. The sludge
accumulation should be monitored regularly to make sure that it does not disrupt the filter's operation : spreading
system, ventilation shafts, water distribution, etc. When the sludge layer is about fifteen centimetres thick, it
should be removed. It can then be reused as agricultural compost.A20 - Macrophyte filters3) Horizontal constructed wetlandsPrinciple
The horizontal filter fed continuously or otherwise by scoops. Its operation does not require a system idle phase.It is thus generally not necessary to install several filters in parallel. Water flows horizontally and is purified by
micro-organisms that attach themselves to the roots of macrophyte plants.There is a very high risk of the filter clogging if the water has not been pre-treated. The horizontal filter should
thus not be used at the start a sanitation system, but should be preceded by a primary treatment system, for
example, a vertical filter or decanter (see fact sheet A19 "Settlers/digesters").This process is especially suitable for effluent containing little solid matter or, for example, for rural or suburban
communities already equipped with small pre-treatment systems (cesspools or septic tanks). On a small scale, this
type of filter can also be used individually (see fact sheet A24 "Use on a family scale of filter ponds planted withFeeding and discharge[/IMG/UserFiles/Images/assainir/art-86-4.jpg]Source : Macrophyte plant group and Water treatment
Cylindrical baskets, containing pebbles and used for the purposes of transition between the filter medium and the
system's pipes, are placed at the filter inlet and outlet. Their purpose is to ensure a water distribution over the wholewidth of the cylindrical basket. A simple system is used to regulate the water level, which should ideally be
maintained 5 to 10 cm below the filter surface.The main filter element : the filter bed[/IMG/UserFiles/Images/assainir/art-86-5.jpg]Discharging system at the outlet of horizontal constructed wetlands,as per EPA, 2000A20 - Macrophyte filters
The filter bed is installed in a hole dug in the ground. It must be sealed to prevent water that has not yet been treated
from seeping into groundwater. It should be about 50 to 60cm deep, in other words, less deep than verticalfilters. It is composed of gravel with a grain size of between 4 and 8 mm. If the treated water contains too many very
fine particles, the filter may become irreversibly clogged.
The bottom of the filter must have a slope of about 5%, to avoid water stagnation.
When the filter is started, 4 macrophyte plants should be planted per m² with their clods. They will multiply during
the filter's operation. Their role is to supply oxygen to the filter bed and allow the fixing of micro-organisms.
Through a series of physical, chemical and biological processes, solid particles settle, organic matter is degraded and
nutrients clinging to particles such as nitrogen and phosphorus are degraded by micro-organisms attached to the
Maintenance
The filter's proper operation should be monitored regularly, once a quarter, for example, to ensure in particular that the
filter does not become saturated and that the drains do not become clogged.
Macrophyte plants should not be cut down. Simply remove dead leaves and branches, which may encourage the
growth of weeds if they remain in the filter.Unlike vertical filters, no layer of sludge forms on the surface.4) Installation example using these two types of filter
This is the system No 3 referred to in fact sheet A17.[/IMG/UserFiles/Images/assainir/art-83-3.jpg]
The horizontal filter needs less of a slope than the vertical filter. Two to three metres level difference is sufficient
for this system. It is important for the water to flow first through the vertical constructed wetland : this stage is needed
to prevent the horizontal filter clogging.This system is more effective than a system involving installing two vertical filters in series to remove germsfrom faeces. It should thus be preferred for black water treatment (domestic wastewater + faeces + urine).
Variant of this system : replacement of the scoop system and the vertical filter by a settler, which allows it to be
installed on a plot of land with a level difference of a metre and be used without a pump on a less sloping plot of land.A20 - Macrophyte filters5) Advantages and disadvantages of constructedwetlandsa) Advantages
- Constructed wetlands require relatively little maintenance.
- They provide an environmentally friendly and more aesthetically appealing solution which, in general, does not
- They are well suited for small towns or suburban areas.
- They are fairly cheap to build and their operating costs are very low.b) Drawbacks
- Constructed wetlands require either a relatively large and slightly sloping plot of land or a pump to raise the
- They are not very effective against nitrification and phosphatation. They also only provide partial treatment andmust thus be associated with other systems.
- They need to be designed by specialists.
- Vertical filters should be fed by a scoop system.
- Horizontal filters may be clogged if the incoming water contains too many solid impurities.
It is particularly relevant to express cost as an order magnitude expenditure per household.
The investment expenditure can be estimated to be Euros30 to 60 per household depending on the system, the
number of users, but only Euros3 or Euros4 per household per year to operate.6) Where to obtain further information - Bibliography
- EPNAC website : The working group on the Assessment of new sanitation processes for small- and medium-sized
communities (GT - EPNAC) Document available online at :
A20 - Macrophyte filters
- Report of the Loire-Bretagne water agency on constructed wetlands and other sanitation methods. Pages 27 to 34.
- EAWAG (Switzerland) "Compendium of water treatment systems and technologies". This comprehensive work
includes several pages on constructed wetlands. Well illustrated and detailed document available (online) at :
- PDM and PSEau SMC Methodological guide (concerted municipal strategies) : "Choosing suitable technicalsolutions for liquid waste disposal", which is a very interesting and informative 136-page illustrated guide produced
by GRET, pages 109 to 115 deal with the issue of sludge treatment.